1. Field of the Invention
The present invention relates to a digital video tape recorder (hereinafter called a digital VTR) having a track format that records digital video and digital audio signals in respectively predetermined areas on a slant track, and more particularly to a digital VTR for recording digital video and digital audio signals inputted as a bitstream.
2. Description of Related Art
FIG. 1 shows a track format employed in a common home digital VTR. As shown, slant tracks are formed on a magnetic tape, and each track is divided into two areas, a video area for recording a digital video signal and an audio area for recording a digital audio signal.
There are two methods to record video and audio signals on such home digital VTRs. One is the so-called baseband recording method in which the video and audio signals are inputted in analog form, and recorded in digital form using video and audio high-efficiency encoders; the other is the so-called transparent recording method in which bitstreams transmitted in digital form are recorded.
For recording of the Advanced Television (ATV) signal currently under consideration in the United States, the latter method, i.e., the transparent recording method, is suitable. Major reasons are that the ATV signal is already digital-compressed and does not require high-efficiency encoders or decoders, and that there occurs no degradation in picture quality since the signal is recorded directly. On the other hand, the major drawback is inferior picture quality in special playback modes such as high-speed playback, still-motion playback, and slow motion playback. In particular, by simply recording bitstreams directly on slant, tracks, useful pictures cannot be reproduced in high-speed playback.
One digital VTR method for recording the ATV signal was proposed in a technical report "A Recording Method of ATV data on a Consumer Digital VCR" presented at "International Workshop on HI)TV '93" held from October 26 to 28 in Ottawa, Canada. This report will be used as the prior art in the following description.
According to the basic specification of a prototype home digital VTR, assuming the recording rate of the digital video signal is 25 Mbps and the field frequency is 60 Hz, one video frame is recorded in video areas on 10 tracks in standard definition (SD) mode. Here, if the data rate of the ATV signal is 17 to 18 Mbps, transparent recording of the ATV signal is possible in this SD mode.
FIGS. 2A and 2B are diagrams showing the head scan paths in normal playback and high-speed playback on a conventional digital VTR. As shown, slant tracks are recorded alternately across the tape by heads having different azimuth angles. In normal playback, the tape transport speed is the same as in recording, so that the heads precisely trace the recorded tracks as shown in FIG. 2A. In high-speed playback, on the other hand, since the tape speed is different, the heads move across several tracks, each head thus being able to play back only fragments of the same azimuth tracks. FIG. 2B shows an example in fast forward playback at the speed five times the normal speed.
In MPEG2 bitstreams (bitstreams of the ATV signal are substantially compatible with MPEG2 bitstreams), only intracoded blocks can be decoded independently without referencing other frames. If an MIPEG2 bitstream is recorded on tracks in successive order, images in high-speed playback will be reconstructed using only intra-codes since high-speed playback data is a burst. In this case, on the screen the reproduced areas will be non-continuous, and fragments of blocks will be dispersed across the screen. Furthermore, since the bitstream is variable-length encoded, there is no guarantee that the whole screen will be updated periodically, and there is a possibility that some portions may remain unupdated for long periods of time. As a result, the picture quality in high-speed playback will not be sufficient and unacceptable for a home digital VTR.
FIG. 3 is a block diagram showing the configuration of a bitstream recording apparatus in a home digital VTR of the prior art. Here, the video area on each track is divided into a main area, where the whole bitstream of the ATV signal is recorded, and a duplication area, where portions of the bitstream critical for image reconstruction in high-speed playback are recorded (such portions are hereinafter referred to as High Priority (HP) data). Since only intracoded blocks are valid in high-speed playback, intra-codeed blocks are recorded in the duplication area; to further reduce the data amount, low-frequency components are extracted from all the intra-coded blocks and recorded as HP data. In FIG. 3, reference numeral 401 is a bitstream input terminal, 402 is a bitstream output terminal, 403 is an HP data output terminal, 404 is a variable-length decoder, 405 is a counter, 406 is a data extraction circuit, and 407 is an EOB (end of block) appending circuit.
An MPEG2 bitstream is inputted via the input terminal 401 and outputted unprocessed via the output terminal 402 for recording successively in the main area. The bitstream inputted via the input terminal 401 is also fed to the variable-length decoder 404, which analyzes the syntax of the MPEG2 bitstream and detects an intra-image; in response, the counter 405 generates the timing at which the data extraction circuit. 406 extracts the low-frequency components of each block of the intra-image. Then, the EOB appending circuit 407 appends an EOB to construct. HP data which is recorded in the duplication area.
FIG. 4 is a diagram conceptually showing the playback operation of the prior art digital VTR. In normal playback, the whole bitstream recorded in the main area is reproduced and supplied to an MPEG2 decoder external to the digital VTR. The HP data is discarded. On the other hand, in high-speed playback, only the HP data recorded in the duplication area is selected and sent to the decoder, while the bit-stream from the main area is discarded.
Next, the arrangement of the main area and duplication area on each track will be described. FIG. 5 is a diagram showing an example of the head scan path in high-speed playback on the prior art digital VTR. If the tape speed is an integral multiple of the normal speed and phase-lock controlled, the head scanning is synchronized with the tracks with the same azimuth, reproducing data always from the same positions. In FIG. 5, if portions of the reproduced signal whose output levels are higher than -6 dB are played back, hatched regions will be played back by one head. FIG. 5 shows an example of high-speed playback 9 times the normal speed. At this 9-times playback speed, it is guaranteed that the signal will be read from the hatched regions. It can therefore be seen that the HP data should be recorded in these areas. However, at other fast playback speeds, there is no guarantee that the signal will be read; the regions need to t)e set so that the signal can be read at different tape speeds.
FIG. 6 shows the track regions that the head scans in high-speed playback on the prior art digital VTR. Some of the regions scanned at various tape speeds overlap. Duplication areas are selected from among these regions to guarantee HP data reading at different tape speeds. Shown in FIG. 6 are examples of fast playback 4, 9, and 17 times the normal speed. The scan areas shown are the same as those that will be selected for fast playback -2, -7, and -15 times the normal speed.
It is not possible for the head to trace exactly the same regions at different tape speeds, because the number of tracks that the head crosses is different at different tape speeds. Furthermore, it is required that the scan areas be traced from any of the same azimuth tracks. FIG. 7 shows examples of head scan paths at different tape speeds in the prior art digital VTR. In FIG. 7, regions 1, 2, and 3 are selected from among the overlapped regions between 5-times and 9-times speeds. By repeating the same HP data on nine tracks, the HP data can be read at both 5-times and 9-times speeds.
FIGS. 8A and 8B show examples of the head scan path in 5-times speed playback on the prior art digital VTR. As can be seen from the figure, by repeating the same HP data over the same number of tracks as the speed expressed as a multiple of the normal speed, the HP data can be read by the head synchronized with the same azimuth tracks. As a result, by duplicating the HP data over the same number of tracks as the maximum tape speed in high-speed playback expressed as a multiple of the normal speed, reading of the duplicate HP data can be guaranteed at different tape speeds in both forward and reverse directions.
FIG. 9 shows a track format employed in the prior art digital VTR; shown here is an example of a main and duplication area layout. In a home digital VTR, the video area on each track consists of 135 sync blocks; in the example shown, the main area consists of 97 sync blocks and the duplication area consists of 32 sync blocks. This duplication area is made Up of the overlapped areas between the 4-times, 9-times, and 17-times speeds shown in FIG. 6. In this case, the data rate of the main area is about 17.5 Mbps, arid that of the duplication area is about 340 Kbps since identical data is recorded 17 times.
FIG. 10 is a diagram for evaluating the quality of the high-speed playback image reconstructed from the HP data created by the above method. As previously noted, the HP data consists only of intra-coded blocks. In high-efficiency encoding schemes employed in MPEG2, etc., a combination of orthogonal transform, based on DCT, and two-dimensional variable-length coding is used as the intra-coding. The following description of the prior art is given, assuming that the DCT and variable-length coding are used as the intra-coding. FIG. 11 is a diagram showing the structure of a DCT block. As shown, the DCT block is obtained as a data block of 8.times.8 terms arranged in horizontal and vertical directions. In the DCT block, the data of sequence number 0 in the upper left corner of the block is called the DC coefficient, and the data of sequence numbers 1 to 63 are called the AC coefficients, of which those having smaller sequence numbers are called low-frequency coefficients and those having larger numbers are called high-frequency components. The coefficients in the DCT block are outputted, starting with the lowest frequency component, in increasing order of the sequence numbers shown in FIG. 11, by a scanning method called zigzag scanning. In FIG. 10, the transmitted data rate (hereinafter called the data acquisition rate) is plotted along the abscissa when the number of sequences transmitted in sequence starting with the DC coefficient is varied, the transmitted data amount being 100% when all the coefficients in the DCT block are transmitted. In FIG. 10, the S/N ratio is also shown plotted along the ordinate as a function of the number of transmitted sequences. FIG. 12 is a diagram for explaining the graph of FIG. 10, showing the number of transmitted sequences corresponding to each of the plotted points (the leftmost point in FIG. 10 is numbered 0).
From FIG. 10, it can be seen that as the number of high-frequency coefficients transmitted increases, the S/N ratio improves markedly for the amount of change in the data acquisition rate. The data shown was obtained by computer simulation performed only on the Y signal in the NTSC format, but it is expected that the same relationship between the data acquisition rate and the amount of improvement in S/N ratio substantially holds for the ATV signal.
The relationship between the data acquisition rate and the S/N ratio when the number of sequences is varied, shown in FIG. 10, will be described below using the example of the main and duplication area layout on each track shown in FIG. 9. As shown in FIG. 9, according to the basic specification of a prototype home digital VTR, the video area on each track consists of 135 sync blocks, and the data rate of the main area is 17.5 Mbps and that of the duplication area is about 340 Kbps. Suppose here that one intra-picture is transmitted per second. Then, the data rate of intra-coded data is estimated at about 1.5 Mbps, so that the data acquisition rate of the duplication area is about 23% of that of the main area (340 K/1.5M=0.23).
With this in mind, referring is made to FIG. 10, from which it can be seen that the DC coefficient and two AC coefficients can be transmitted before the data acquisition rate reaches about 23%. Therefore, the S/N ratio of the reproduced image in high-speed playback can be estimated at about 23.9 dB.
FIG. 13 is a diagram for explaining the prior art method of recording the HP data, showing a specific example of how the HP data is recorded in the duplication area on each track when the maximum tape speed in high-speed playback is 17 times the normal speed, as in the example shown in FIG. 9. In the figure, A and B indicate the tracks recorded by heads having different azimuth angles. The numbers 1, 2, 3, shown in the figure indicate the positions of the HP data on the screen. By taking the HP data positions 1, 2, and 3 on the screen as an example, reproduction of the HP data will be described below.
FIG. 14 is a diagram showing the head scan path in high-speed playback 5-times the normal speed. Shaded portions shown in the figure are the regions in the same azimuth playback areas from which high-speed playback data is obtained in high-speed playback 5-times the normal speed. As shown, head A scans the HP data regions in the same azimuth playback areas in the order of 1, 2, 3, 1, 2, 3, and so on. On the other hand, head B scans the HP data in the same azimuth playback areas in the order of 1, 2, 3, and so on. As a result, all HP data at 1, 2, and 3 are reproduced.
FIG. 15 is a diagram showing the head scan path in high-speed playback 9-times the normal speed. Shaded portions shown in the figure are the regions in the same azimuth playback areas from which high-speed playback data is obtained in high-speed playback 9-times the normal speed. As shown, head A scans the HP data regions in the same azimuth playback areas in the order of 1, 2, 3, and so on. On the other hand, head B scans the HP data in the same azimuth playback areas in the order of 1, 2, and so on. As a result, all HP data at 1, 2, and 3 are reproduced.
FIG. 16 is a diagram showing the head scan path in high-speed playback 17-times the normal speed. Shaded portions shown in the figure are the regions in the same azimuth playback areas from which high-speed playback data is obtained in high-speed playback 17-times the normal speed. As shown, head A scans the HP data regions in the same azimuth playback areas in the order of 1, 2, 3, and so on. As a result, all HP data at 1, 2, and 3 are reproduced.
According to the above data construction method that makes high-speed playback possible at three different speeds, 5 times, 9 times, and 17 times the normal speed, the HP data regions 1, 2, and 3 are respectively scanned once, as shown in FIG. 16, in 17-times speed playback, the maximum tape speed in the high-speed playback mode, thereby reproducing all data at 1, 2, and 3 from the same azimuth playback areas. On the other hand, in the 9-times speed playback shown in FIG. 15, the HP data regions 1, 2, and 3 are scanned once to reproduce all data at 1, 2, and 3 from the same azimuth playback areas, and in addition to that, the HP data regions 1 and 2 in the same azimuth playback areas from which the playback signal is obtained, are scanned once again. Similarly, in the 5-times speed playback shown in FIG. 14, the HP data regions 1, 2, and 3 are scanned once to reproduce all data at 1, 2, and 3 from the same azimuth playback areas, and in addition to that, the HP data regions, 1, 2, and 3, in the same azimuth playback areas from which the playback signal is obtained, are scanned two more times.
Some users may want to have rough picture content at a higher playback speed than the predetermined fast playback speed at some sacrifice of picture quality. In the above prior art, a constant picture quality can be provided in playback within the predetermined speed, but it is difficult to provide playback at a higher speed than the predetermined speed. In an MPEG2 bitstream, blocks are arranged in successive order, and the address indicating the block position on the screen is determined in relative manner among the blocks. Therefore, if some blocks are dropped, the remaining blocks cannot, be displayed at their correct positions on the screen. To achieve playback at a higher speed than the predetermined fast playback speed at some sacrifice of picture quality, the number of playback blocks must be increased in exchange for picture quality so that the picture content can be identified. At the same time, for playback within the predetermined fast playback speed, a certain degree of picture quality (exactly equal to or indistinguishable from that achieved in the above-described example) must be provided.
According to the configuration of the above prior art digital VTR, the HP data is recorded by extracting lower-frequency coefficients from the intra-coded blocks; however, as shown in FIG. 10, an adequate S/N ratio cannot be obtained with the low-frequency coefficients alone, but higher-frequency coefficients are needed to obtain a high-speed playback image of high quality.
The configuration of the above prior art home digital VTR has had the further problem that since data for special playback modes is duplicated many times in the duplication areas, the recording rate of the data for special playback modes is extremely low, resulting in insufficient playback picture quality especially in slow-motion or high-speed playback. For example, if the number of intraframes is two per second, the amount of the intra-coded data alone in the ATV signal will be about 3 Mbps; in the prior art, however, data can be recorded only at about 340 Kbps, resulting in significant degradation in playback picture quality.
Furthermore, in the prior art, each track is divided into the main area for normal speed playback and the duplication area for high-speed playback, and the HP data is duplicated on the same number of tracks as the predetermined maximum tape speed in high-speed playback expressed as a multiple of its normal speed, to ensure that the duplicated HP data is played back at least once during playback at several different tape speeds within the maximum playback speed (either in the forward or reverse direction). However, since the same HP data is duplicated the same number of times as the maximum tape speed in high-speed playback expresses as a multiple of its normal speed, in high-speed playback at a slower speed than the maximum tape speed the reproduction efficiency of the HP data actually obtained as the playback signal is low compared with the data amount of the playback signal that could be obtained from the same azimuth playback areas, as shown in the examples of FIGS. 14 and 15. This has presented the problem that the reproduced picture quality degrades because of the decrease in the reproduction efficiency.
In the configuration of the above prior art home digital VTR configuration, since the output level margin for each sync block in the duplication area gradually decreases with increasing playback speed, if 17-times speed playback is performed with the duplication area arrangement shown in FIG. 9, for example, a situation can occur, depending on the head or tape conditions, where the output level of the sync blocks such as the sync block Nos. 16 and 26 is not sufficient, leading to the inability to read data correctly. The problem here is that the block data recorded in the sync blocks that cannot be read out at all. There is the further problem that in order to check all the intra-data, all the bitstream headers of the ATV signal have to be analyzed for every macroblock.